9a9.Gen.capability Curve(Ani.)Mppgcl

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GENERATOR OPERATION LIMITS

By Pradeep Yadav,

Dy. Director.


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When a manufacturer designs & construct an

alternator, It will specify the MCR ( max.

continuous rating )of the machine in terms of;

1. Stator current. (9050A)

The operation of the generator must be such that

none of the design limitations are exceeded as

otherwise damage to the generator may result.

2. Stator voltage. (15.75KV)

3. Rotor current. (2600A.DC)

4. Rotor voltage. (310V.DC)5. MW. (210MW)

6. MVA. (247MVA)

7. Power factor. (0.85 lagging)


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There are 5 principal limitations on the

generator operation.

Principal limitations:

A convenient way of presenting these

limitations is in the form of a diagram

called as Capability curve.

1. Stator current limit.

2. Rotor current limit.

3. MW limit.

4. Stability limit.

5. Stator end heating limit.


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Assumption: Generator terminal voltage is

maintained constant

Erect a voltage vector OA.

Capability curve:


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Principal limitations:

1. Stator current limit : Heat developed in the

winding is limited by the class of Insulation.

At higher current, overheating may damageinsulation.

Class B Insulation - 130 Deg. Cel.

Class F or H - 180 Deg. Cel.


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2. MW limit

The Max. continuous MW rating is

dictated by the steam generation

capability of boiler the H.P rating of the

turbine.

In theory, generators are capable of

generating an output equal to MVA rating.

But generators are designed to operate at a

lagging p.f. ( 0.85 ). So that max.output is

achieved at max.stator current at rated p.f.

MW = MVA

X

Cos =MVA at unity p.f.

= V

X

I

X

Cos


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A horizontal line line drawn from point B

will give MW limit boundary


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3. Rotor current limit:

Same reasoning as in case of stator current

limit.

Short Circuit Ratio( SCR ) is the ratio of field

current required to induce rated nominal

voltage in the Gen.terminal in open circuit

condition

TO

the field current required to circulate rated

stator current in the Gen. terminals with the

terminals short circuited.


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Then, GB will represent full load rotor current or

total field current. With GB as radius , draw an

arc. It will be locus of rotor current and its limit.

Angle between GO GB will be Rotor angle or

stability angle ().

Select a point G such that OG/OB = SCR of

the generator.


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4. Stability limit/Minimum rotor current

limit:

For every load condition of the generator, a

minimum rotor current has to be supplied

to maintain magnetic coupling in the air

gap between stator rotor.A reduction in

field current without any change in steam

admission will lead to break in magnetic

coupling ( increase in rotor angle) with the

subsequent loss of machine stability.

Rotor angle or stability angle (

) < 90for

stable Gen. Operation.


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5. Stator end heating limit:

With high values of stator

current at leading power factor , heating

of the end sections of the stator core tend

to occur.

The end heating limit would normally only

be reached if the stability limits were

exceeded.

Coolent: Normal Limit Property

Cold Hydrogen gas temp. 44 Deg.cel 55Deg.cel 3.5Kg/cm2. 99% pure

Stator water temp. 45 Deg.cel 48 Deg.cel 27m3/hr.


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Rotor angle ()= 90 - Theoritical stability limit.

Practical stability limit () = 75.


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1.Variation of terminal voltage:

Voltage(KV) 14.18 14.96 15.75 16.54 16.7 16.85 17.01 17.17 17.32-------------------------------------------------------------------------------------------------

Stator Current(KA)9.5 9.5 9.05 8.6 8.37 8.14 7.92 7.56 7.24

2.Frquency variation: +5%, -5%

3. Overloading:

Stator current(KA) 9.95 10.41 10.86 11.31 11.76 12.22 12.67 13.57

Rotor current(KA) 2.75 -- -- --- 3.12 -- 3.9 5.2

Time ( Sec. ) 3600 900 360 300 240 180 120 60

4.Operation at different Hydrogen pressure:

Hydrogen Pr.(Kg./Cm2) M.W. Duration

3.0 200 Continous

2.5 170 Not more than 5 Hrs.

2.0 140 -------do------

Operation in air medium is not permitted


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6.One gas cooler out of service175 MW continously.

More than one gas cooler outoperation not permitted.

Coolent: Normal Limit Property

Cold Hydrogen gas temp. 44 Deg.cel 55Deg.cel 3.5Kg/cm2. 99% pure

Stator water temp. 45 Deg.cel 48 Deg.cel 27m3/hr.

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9a9.Gen.capability Curve(Ani.)Mppgcl